The present invention relates generally to nerve electrodes, and more particularly to an improved circumferential neural (circumneural) electrode assembly for implantation on and electrical stimulation of selected nerve tissue of a patient, the electrode assembly providing reduced nerve constriction, improved tissue compatibility, and reduced current spread, and being configured and fabricated for ease of implantation.
Circumneural electrodes are generally designed to encompass a portion of a nerve longitudinally to permit electrical stimulation of the nerve. The stimulation may be intended to modulate electrical signals or impulses normally carried by the nerve. Alternatively or additionally, the nerve electrode may be used for sensing electrical signals carried by the nerve. The required installation of the electrode on a nerve for such purposes presents a considerable number of design problems. To provide mechanical stability of the electrode relative to the nerve, and in recognition that the nerve can move relative to the surrounding tissue, a structure that encompasses the nerve is desirable. This type of structure also provides efficiency in minimizing or optimizing the distance between the stimulating electrode and the nerve body. Nerves, however, are sensitive and easily damaged or traumatized by abrasion or stresses caused by subjection to mechanical forces.
From a mechanical perspective, an ideal peripheral nerve electrode has a structure strong enough to resist tensile forces arising from the attached conductor cable, but pliant enough to prevent tension, compression or constriction of the nerve. Tensile forces acting on the electrode should be minimized to prevent excess nerve constriction or to prevent the electrode from dislodging from the nerve. In addition, circumferential electrodes should be designed to fit closely against the nerve, and yet minimize constriction of the nerve attributable to swelling of the nerve inside the electrode structure.
Adverse mechanical forces can be attributable to constriction of the nerve by the circumneural electrode, or to a pull on or torque transmitted to the electrode (and thus, to the nerve) by a lead wire. Or the nerve may atrophy as a consequence of lack of nutritional fluid exchange owing to the close proximity of the electrode. Previously popular cuff electrodes have lost appeal because of their stiffness that often causes nerve damage.
Present-day nerve electrodes have been found difficult to install on the nerve using common surgical tools. In particular, many designs do not lend themselves to placement using endoscopic tools. Also, difficulty is encountered in explanting the electrode, because of tissue in-growth.
U.S. Pat. No. 4,573,481 discloses an implantable helical electrode assembly with a configuration having one or more flexible ribbon electrodes. Each ribbon electrode is partially embedded in aperipheral surface of an open helical dielectric support matrix adapted to be threaded or wrapped around a selected nerve or nerve bundle during surgical implantation of the electrode assembly. The resiliency of the assembly allows it to expand in the event of swelling of the nerve. The electrode""s expansion characteristic conceptually allows for implantation on a range of nerve sizes. Its resiliency also allows fluid exchange between the helical coils, and mechanical compliance at its ends. But the electrode is difficult to install on the nerve because the helical configuration must first be unraveled and then re-formed about the nerve. In addition, the open structure of the electrode allows for wide current spread between the anode and cathode, which can cause adverse muscle or external tissue stimulation. Further, this electrode is one of those that is difficult to explant or remove from the nerve due to tissue in-growth into the helical structure. Lastly, the complex spiral shape of the electrode renders it difficult to manufacture, lending it to neither automated manufacturing or molding methods.
An improvement in electrode design is disclosed in U.S. Pat. No. 4,920,979, where a flexible electrode-supporting matrix has two oppositely directed helical portions centrally joined and with free outer ends. The helical portions extend circumferentially at least one turn and up to as much as about two turns. A thin, flexible conductive ribbon is secured to the inner surface to provide multiple electrodes on one or both portions, and a connecting electrical cable couples the electrode array to an electronics package for stimulation and/or sensing. The central passage through the helical portions accommodates a pair of pins that extend from the respective closed legs of a tweezer-like installation tool. When the pins are inserted through the central passage and the legs of the tweezers are opened, the helical portions are spread open to allow the assembly to be slipped over the nerve with the two open-sided portions restrained in a direction generally perpendicular to the length of the nerve. Upon release by withdrawing the pins of the installation tool, the two end portions return to a helical shape to encircle the nerve with their electrode portions conductively contacting the nerve surface. This arrangement simplifies electrode installation and reduces nerve trauma during implantation. However, the particular design exhibits poor mechanical retention properties that render the electrode structure easily dislodged from the nerve during implant.
Thus, the availability of these and other circumneural helical or spiral electrodes has not eliminated problems in installation of the electrode on the nerve or in attachment of the lead wire to the electrode assembly. Conflicting design goals of maximizing mechanical strength for fatigue resistance while minimizing spring constant to allow compliance with the nerve and its movement, must be addressed. It is desirable to improve the strength, durability, flexibility and fatigue resistance of the electrode assembly itself, and as well, to improve the mechanical strength of the electrical connection between the lead conductor and the electrode assembly.
In another prior art implantable lead for nerve stimulation, the lead body comprises an MP35N (cobalt chromium alloy) electrical coil conductor having a helical or spiral electrode assembly at its distal end. The conductor has a biocompatible electrically insulative sheath, and a lead connector at its proximal end for insertion into a mating electrical connector of the implanted signal generator. The electrode assembly includes one or more single turn platinum spiral electrical stimulation ribbon conductors with a 90% platinum/10% iridium alloy wire reinforcing component in the weld between the conductor coil and the ribbon electrode. The ribbon conductor is molded in a silicone elastomer insulating material so that the conductor portion is bonded to the insulation but exposed at the underside of the spiral. An integral anchor tether is employed to retain the implanted electrode in place without undue flexing, thereby substantially reducing the possibility of fatigue and fracture of the electrode or the weld connection to the conductor coil.
In the latter prior art design, heat treating of the platinum ribbon stimulating electrode surface makes the platinum material soft and ductile, creating vulnerability of the ribbon electrode to damage during implantation if subjected to excessive manipulation or improper handling. If the electrode helix is overly stretched by the surgeon during installation of the electrode on the nerve, it may be deformed to an extent that affects its performance and long term reliability as a nerve-stimulating electrode. The tether reduces the magnitude of repetitive, small force loads on the lead connection to the electrode after implantation, but the possibility of mechanical fatigue at the weld joint remains.
The assignee of the invention disclosed in the present application owns several improvement patents covering nerve electrodes, including U.S. Pat. Nos. 4,979,511; 5,215,089; 5,251,634; 5,351,394; and 5,531,778. The ""511 patent discloses the feature of a strain relief tether for an implantable nerve electrode. The ""778 patent discloses a nerve electrode having a ribbon conductor portion composed of platinum-iridium alloy that is stronger than prior types because it is neither heat treated nor annealed. The electrode is preferably made from a platinum-iridium alloy ribbon rather than reinforcing platinum with another Pt/Ir ribbon since the Pt will anneal from the weld heat. Thinner ribbon improves flexibility and elastic memory of the electrode assembly, allowing the electrode to be opened and placed on the nerve during implantation with greater assurance that, upon closing, it will return to its original shape without distortion. The number of turns of the helix is reduced from 3 to 2-xc2xd to simplify installation of the electrode on the nerve, while maintaining an adequate number of turns for attachment.
In the latter design, the strength of the coil/ribbon weld junction is improved by virtue of the alloy""s retention of mechanical strength in the weld area. Both ribbons are 0.013 mm thick, totaling 0.026 mm. Further improvement in the weld junction is achieved by welding the conductor coil of the lead directly to the reinforced ribbon, which improves the mechanical interlocking of the coil and ribbon structure into the silicone elastomer. The effect is to reduce stress transmitted directly from the coil to the ribbon, and prevent separation of the silicone elastomer from inside the welded ribbon area. Silicone elastomer is encapsulated inside a cylinder formed by the ribbon loop, in which the weld area resides. Swelling of the elastomer in body fluids increases mechanical stability inside the weld loop to preclude or minimize tissue in-growth into that area.
The electrode assembly of the ""778 patent includes a flexible electrically insulative carrier of helical configuration, a flexible ribbon electrode secured to the underside of at least a segment of the helical configuration, an elongate conductor for electrical connection to the ribbon electrode, and a flexible conducting spacer electrically connected to the ribbon electrode and to the elongate conductor for separating the latter from the helical configuration while maintaining electrical connection between the two at the distal end of the conductor. The distal end of the lead projects directly and tangentially from a curved portion of the spacer in a direction substantially parallel to the longitudinal axis of the helical configuration.
It is a principal aim of the present invention to provide an electrode assembly for nerve stimulation, with improvements in resiliency of its electrode structure to reduce constriction of the nerve, in tissue compatibility, and in reduction of current spread, and which is more easily implanted on the nerve.
Another aim is to provide a nerve electrode assembly that offers improved retention on the nerve while it reduces constriction of the nerve, relative to prior nerve electrodes.
The present invention provides a nerve electrode design comprising a flexible electrode-supporting matrix that has one or more circumferential metallic electrodes centrally joining a central spine or elongate conductor. Helical portions extend circumferentially up to one turn or 360 degrees. A thin, flexible conductive ribbon is secured to the inner surface of the matrix to provide multiple electrodes on one or both portions, and a connecting electrical cable couples the electrode array to an electronics package for stimulation and/or sensing. One feature of the design is the ability of the electrode to elastically expand along the circumference of the electrode. The flexible conductive ribbon material is connected to a thin resilient material which stretches under small stresses. The ability of the electrode to stretch prevents nerve constriction and nerve damage. The amount of elasticity and expansion needed to accommodate swelling of the nerve can be controlled by material selection and thickness of material. For example, silicone rubber is one material which has good elastic properties. One approach to produce the desired elastic electrode structure is to use a thinly folded material such as polytetrafluoroethylene (PTFE) which will allow for expansion and stretching of the electrode along the circumference. Another approach is to fabricate the electrically conductive element from a conductive fabric made from fibers such as polyester that are sputter coated with a conductive metal coating. Using this approach a conductive, elastic fabric is formed by essentially combining conductive fibers with elastic fibers. The conductive fibers or coating on elastic fibers may be composed of any one of the precious metals or their oxides or alloys thereof, or of carbon or graphite. Conductive elements may also be made of braided or mesh materials.
The electrode may consist of one conductor (monopolar) or any number of conductive elements. To prevent current spread, the electrode has a cuff configuration. The cuff material should be electrically insulating, and possess properties of thinness and resiliency to enable it to conform closely to the shape of the nerve and be less constrictive than current designs, as well as to minimize forces and stress concentrations imparted by it to the nerve. The material is biocompatible and preferably porous, composed, for example, of a polymer such as PTFE or polyester. Or a silicone elastomer may be used, with porosity provided by piercing the material to form small holes in it, typically of a size less than 5 microns. The porous insulation allows for fluid and ion transfer to the nerve, and nourishment and pore size is minimized to prevent tissue ingrowth. Tissue ingrowth into the longitudinal seam is prevented by cuff overlap. Ideally, the insulting material should have viscoelastic properties and the materials elastic modulus should be compatible with the surrounding tissues.
The simple cuff geometry of the nerve electrode design of the present invention offers the advantage of ease of implantation. The electrode can be easily placed onto the nerve endoscopically using common surgical instruments such as miniature forceps. The surgical procedure consists of passing the electrode in a protective cylindrical carrier to a site adjacent to the isolated nerve attachment site. The electrode then is removed from the carrier and grasped on each of the free ends of the electrode structure. The electrode is then placed around the nerve and secured by hooking the fastener. Alternatively, the electrode can be tied together at the ends by using elastic ties. The ties are made from silicone rubber or other resilient material which allows for stretching due to nerve swelling or tensile forces acting on the electrode structure from the cable.
The conductive element is preferably composed of a thin metallic ribbon or small diameter metallic fibers, of an inert material such as one of the platinum group metals or one of its oxides such as iridium oxide or rhodium oxide. The surface area of the electrode is chosen to prevent excessive dissolution, and the electrode is designed to operate within safe charge injection limits for the material chosen. The conductive elements may be cathodes, anodes or any combination of cathode/anode. The conductive elements are preferably spaced apart a distance of at least one diameter to prevent current shunting between the cathode and anode.
A method of reducing tensile forces imparted to the electrode is discussed in the aforementioned ""511 patent, but the tether design of that electrode suffers some installation difficulty owing to multiple helical turns. An embodiment of the electrode of the present invention incorporates a service loop built into the coil to absorb stresses from the cable and minimize stress at the electrode structure that is located around the nerve. A feature of the service loop is a thin tear away webbing that extends between adjacent serpentine undulations of the cable to prevent tissue ingrowth therebetween. This creates a planar section which is unobstructed by tissue, and the tear away webbing feature allows the cable to extend, under tensile stresses, by pulling through the tissue. This feature may be incorporated along the lead body to allow for extension of the lead under typical stretching of the lead body which can occur during normal body movements or due to growth of younger patients.
Prior art cuff electrode designs have had a disadvantage of nerve constriction due to swelling of the nerve from trauma of the implantation procedure, which can cause nerve damage. The design afforded by the present invention incorporates a flexible material such as a thin layer of PTFE or fabric material which insulates the metallic conducting elements and minimizes nerve constriction. The circumneural electrode sheath can expand independently from the metallic stimulating electrodes.
Previous cuff electrodes have also suffered from designs that do not allow fluid or ion exchange across the insulator material. This can cause poor ion exchange, electrolyte imbalance, and entrapment of residual by-products due to electrochemical stimulation reactions. The present design incorporates a porous material such as a thin layer of silicone rubber, PTFE or fabric material that insulates the metallic conductors and allows fluid and ion exchange.
Additional advantages of the design of the present electrode include the following:
(i) ease of placement on a nerve with common surgical tools because it does not encircle the nerve for multiple turns, and ease of fixation to the nerve;
(ii) ease of explantation since it only encircles the nerve one revolution and the continuous sheath insulator cuff can be removed in one piece;
(iii) insulated between cathode and anode, so as to limit the stimulus current spread to adjacent tissues;
(iv) readily fabricated using automated manufacturing methods;
(v) incorporates elastic closure ties or straps which are designed to keep the electrode encircled around the nerve but allow for expansion due to swelling;
(vi) circumneural electrode sheath can expand independently from the metallic stimulating electrodes;
(vii) electrode materials selected to match tissue compliance and elastic modulus;
(viii) incorporated medication in or coated surface of electrode to minimize tissue inflammation, improve tissue compatibility, lower stimulation threshold, and/or prevent tissue adhesion;
(ix) electrode structure of woven polymeric fabric incorporates metallic fibers or metallic coating for strength, thinness and elasticity, for conformance to the nerve and matching to tissue modulus and compliance, and prevention of tissue in-growth.